1. Field of the Invention
The present invention relates generally to an apparatus and method for receiving data in a mobile communication system. More particularly, the present invention relates to an apparatus and method for receiving packet data in a mobile communication system.
2. Description of the Related Art
Mobile communication systems have been developed to provide voice service, while guaranteeing mobility of the users. With the rapid development of communication technologies, the mobile communication systems can now support data service. Recently, many researches are being conducted for high-speed data transmission in a Code Division Multiple Access (CDMA) mobile communication system. A CDMA2000 1x Evolution Data Only (1xEV-DO) system is a typical mobile communication system having a channel structure for the high-speed data transmission. The CDMA2000 1xEV-DO system has been proposed in Third Generation Partnership Project 2 (3GPP2) to make up for defects of data communication in the IS-2000 system.
In the 1xEV-DO system, data communication can be divided into data communication in a forward direction and data communication in a reverse direction. Herein, the term “forward direction” refers to a direction from an access network (or base station) to an access terminal (or mobile station), and the term “reverse direction” refers to a direction from an access terminal to an access network.
To maximize forward transmission power, the 1xEV-DO system generally has an architecture in which an access terminal measures minimum Carrier-to-Interference ratio (C/I), which is a forward channel environment parameter, and an access network transmits data using a Data Rate Control (DRC) value, which is a forward data rate determined by the access terminal.
FIG. 1 is a diagram schematically illustrating a general CDMA2000 1xEV-DO system. Reference numeral 100 denotes access terminals (ATs), reference numeral 110 denotes access networks (ANs, also known as an access points (APs)), and reference numeral 120 denotes access network controllers (ANCs). A brief description will now be made of the system architecture. A first AN 110a is shown to perform communication with a plurality of ATs 100a and 100b, and a second AN 110b is shown to perform communication with another AT 100c. The first AN 110a is connected to a first ANC 120a, and the second AN 110b is connected to a second ANC 120b. Each of the ANCs 120a and 120b can be connected to two or more ANs. For convenience, it is shown in FIG. 1 that one ANC is connected to one AN. Each of the ANCs 120a and 120b is connected to a Packet Data Service Node (PDSN) 130 that provides packet data service, and the PDSN 130 is connected to an Internet network 140.
In this mobile communication system, each of the ANs 110a and 110b transmits packet data only to the AT having the highest packet data rate among the ATs communicable therewith, that is, the ATs located in its coverage. A detailed description thereof will now be given. In the following description, an AT will be denoted by reference numeral 100, and an AN will be denoted by reference numeral 110.
For forward channel rate control, an AT 100 measures received power of a pilot channel transmitted by an AN 110, and determines a forward data rate desired by ATs 100 according to a predetermined fixed value based on the measured received pilot power. Thereafter, the AT 100 transmits DRC information corresponding to the determined forward data rate to the AN 110 over a DRC channel. Then the AN 110 receives DRC information from all ATs located in its own coverage. Thereafter, based on the DRC information, the AN 110 can transmit packet data to a particular AT having a good channel state at the data rate reported by the AT. Herein, the DRC information refers to a numerical value determined by the AT by measuring its own channel state and determining an available forward data rate depending on the measured channel state. Although a mapping relationship between the forward channel state and the DRC information is subject to change according to implementation, a value fixed in the AT manufacturing process is generally used as the mapping relationship.
The CDMA2000 1xEV-DO system has evolved into a Revision A system. The Revision A system aims at efficiently providing not only high-speed data service but also a service that needs to guarantee Quality-of-Service (QoS), like voice packets. Accordingly, a Multi-User Packet has been newly added in addition to a conventional Single-User Packet as a forward transmission scheme. Multi-User Packet, which has been proposed to prevent unnecessary traffic waste when transmitting data to one AT at a certain time, is designed to transmit packets to a plurality users. A brief description will first be made of frame structure, Single-User Packet and Multi-User Packet in the CDMA2000 1xEV-DO system, and thereafter, a detailed description will be made of a method for determining a data rate in the CDMA2000 1xEV-DO Revision A system.
FIG. 2 is a diagram illustrating a frame structure in the CDMA2000 1xEV-DO system. Referring to FIG. 2, a frame 201 of a forward channel is comprised of 16 slots and has a length of about 26.666 msec. Each slot of the frame 201 is divided into two half slots 203 and 205 before being transmitted to prevent transmission error. Each of the half slots 203 and 205 has a length of 13.33 msec and is comprised of 1024 chips.
Because the half slots 203 and 205 are identical to each other, only the first half slot 203 will be described. The first half slot 203 is comprised of Medium Access Control (MAC) channels 209 and 213, a Pilot channel 211, and Data channels 207 and 215. The MAC channels 209 and 213 each have a 64-chip length and take charge of controlling radio capacity and controlling transmission power of each AT. The Pilot channel 211 is comprised of 96 chips and used for synchronization, decoding, demodulation and C/I measurement for each AT. That is, the AT determines a rate of forward data from a C/I value of the Pilot channel 211, and sends a request for determined data rate to an AN. Finally, the Data channels 207 and 215 are channels containing actual traffic. Every slot has two Data channels, and each Data channel is comprised of 400 chips. The AN includes preambles in the Data channels 207 and 215 containing actual traffic during initial transmission of each frame.
Next, with reference to FIGS. 3A and 3B, a description will be made of structures of the Single-User Packet and Multi-User Packet for the Data channels 207 and 215. Although the data channels are represented as the different reference number, the data channels can be either a Single-User Packet or a Multi-User Packet.
FIG. 3A is a diagram illustrating a format of the Single-User Packet data. Referring to FIG. 3A, the Single-User Packet data, which is transmitted when the number of ATs receiving transmission packets from a physical layer is one, is divided into a Preamble 301 and a Traffic field 307. The Traffic field 307 is divided into a Single-User payload 303 and a Trailer 305. The Preamble 301 includes such information as an identifier (ID) of a receiving AT, and the receiving AT determines whether the information is its own information using the Preamble information, and performs demodulation using a transport format corresponding to a DRC value appropriate to itself. The Traffic field 307 includes the traffic to be actually transmitted by the AN, and the Trailer 305 includes a Cyclic Redundancy Code (CRC), and is used for determining whether there is any error in a received signal.
FIG. 3B is a diagram illustrating a format of Multi-User Packet data. Referring to FIG. 3B, the Multi-User Packet data is also divided into a Preamble 311 and a Traffic field 319. The Traffic field 319 is comprised of a Header 313, Multi-User payload 315, and Trailer 317. The Preamble 311 of the Multi-User Packet data, as Multi-User Packet transmits data to a plurality of users using one packet, includes the information indicating whether the currently transmitted packet is a Multi-User Packet based packet, and the packet size information. The information on IDs of several ATs and a number of users are included in the Multi-User payload 315.
With reference to FIG. 4, a description will now be made of a method for controlling a forward data rate in the CDMA2000 1xEV-DO system employing Single/Multi-User Packet.
FIG. 4 is a diagram illustrating a connection between an AN 401 and an AT 410 in the general CDMA2000 1xEV-DO network. Herein, the AN 401 represents a base station in the general mobile communication system, and the AT 410 represents a mobile station in the general mobile communication system. For convenience, a description of undepicted elements of the AN 401 will be omitted for clarity and conciseness. A brief description will now be made of constituent elements.
Referring to FIG. 4, in the CDMA2000 1xEV-DO system, a decoder 411 and a C/I estimator 413 of the AT 410 receive a signal transmitted through a forward link channel 430. The C/I estimator 413 estimates a C/I value by measuring a pilot value of the received signal. The decoder 411 transmits Packet Error Rate (PER), or Packet Error Event information of the received signal to an Automatic Repeat reQuest (ARQ) unit 417 and a threshold setting unit 419. The ARQ unit 417 provides information indicating whether to perform an ARQ operation to the AN 401 through a reverse link channel 440 according to the PER determined by the decoder 411.
The threshold setting unit 419 sends a threshold to a DRC decision unit 415 so that the DRC decision unit 415 may receive a PER from the decoder 411 and determine a data rate according to the C/I. Thereafter, the DRC decision unit 415 transmits the determined rate information to the AN 401 over the reverse link channel 440 along with an ARQ message.
Then an ARQ decoder 405 and a DRC decoder 407 of the AN 401 decode the ARQ message and DRC information received through the reverse link channel 440, respectively, and transmit the decoded information to a scheduler 403. Thereafter, the scheduler 403 allocates forward resources and performs an operation according thereto.
In a Single-User Packet decoding method, because a preamble of Single-User Packet data includes an ID of a receiving AT, the AT 410 determines whether received information is its own information using the preamble information, and performs decoding using a transport format corresponding to a DRC value transmitted by the AT 410 itself. However, in a Multi-User Packet decoding method, a preamble of Multi-User Packet data includes the information indicating whether the currently transmitted packet is a Multi-User Packet based packet, and the packet size information. Therefore, upon receiving Multi-User Packet data, the AT 410 compares packet size information included in the preamble with payload sizes of transport formats indicating a plurality of Multi-User DRC values mapped to the DRC value transmitted by the AT 410 itself. After checking the preamble, if there is any received data whose payload size is identical to the packet size, all ATs 410 perform decoding according to the transport format using their own decoders 411. That is, the AT should succeed in decoding the preamble in order to determine whether its own information is included in the current packet. However, if there is a preamble error in the Multi-User Packet data, the AT may undesirably decode the traffic field using a misaligned transport format. Therefore, in the preamble check process, even in a possible early termination situation, the AT 410 repeatedly receives data, unnecessarily wasting the radio resources. This causes a reduction of the total system performance.
Accordingly, there is a need for an improved apparatus and method for efficiently receiving packet data in a mobile communication system.